To establish a perfect Ag stress migration bonding, the detailed growth process of hillocks and grains in Ag films deposited on Si substrate has been investigated by cross-sectional analysis of nanoscale grains. The instability in the microstructural evolution depends on the process temperature; hillock growth is dominant up to 300°C, while lateral grain growth becomes superior at higher temperatures. Theoretical analysis of the results reveals that the growth mode is governed by grain boundary diffusion along the columnar grains. Our finding supports that Ag stress migration bonding can be an attractive bonding technology for various applications in highly reliable advanced devices.